The present invention relates to methods and apparatus for sensing leakage current in a system, and more particularly, to a circuit that is capable of sensing such leakage and also operates to perform self-testing to ensure proper sensing capability.
In alternating current (AC) power circuits, current from a single phase AC source normally flows from an AC source via a phase wire, through a load that is fed by the AC power, back through a neutral wire to the AC source, and vise verse. Any component of current that does not flow to the load via the phase wire and from the load via the neutral wire will flow via one or more leakage paths to earth ground. Such current is known as leakage or residual current. Leakage current is dangerous because it can lead to electrocution and/or fires if not properly controlled.
Ground Fault Interrupters (GFIs), which are also known as Residual Current Detectors (RCDs), are commonly available devices for detecting and interrupting leakage current flow. A diagram of a typical GFI circuit 10 is shown in FIG. 1. By way of example, a single phase AC source of power is provided by way of a phase wire L and a neutral wire N. The phase and neutral wires L, N are coupled to the load 14 (which in a residence may be an appliance of some kind), and AC current cycles from the source over the phase wire L, through the load 14, and back to the source via the neutral wire N, and vise verse.
The GFI circuit 10 includes a differential current transformer (T1), including a toroidal core through which the phase and neutral wires L, N pass. By passing through the current transformer, T1, the phase and neutral wires L, N function as the primary winding of T1. The secondary winding 12 includes a number of turns of wire wound around the core of the transformer T1. The secondary winding 12 is coupled to a circuit interrupter 16. The circuit interrupter 16 includes a control function 16A and a switch 16B, which is illustrated as a double pole, single throw (DPST) switch. The control function 16A operates to actuate (open) the DPST switch 16B when there is voltage present across, and/or current through, the secondary winding 12.
When there is no leakage current to earth ground, the sum of the respective AC currents in the phase and neutral wires L, N (through the primary winding) of the transformer T1 is zero. This results a net zero current flow in the primary of the transformer T1, which in turn results in no induced current in the secondary winding 12. Thus, the control function 16A does not open the DPST switch 16B and the AC current continues to flow through the load 14 from the AC source.
When there is leakage current to ground, however, the sum of the respective AC currents in the phase and neutral wires L, N (through the primary winding) of the transformer T1 is not zero. Indeed, for example, if some current were to flow from the AC source over the phase wire L, through earth ground, and back to the AC source, effectively bypassing the transformer T1, then there would be a greater magnitude AC current flowing over the phase wire L, through the primary of the transformer T1, than would return over the neutral wire N, through the primary of the transformer T1. The imbalance in current through the primary winding of the transformer T1 caused by the leakage current flow induces a current in the secondary winding 12 of the transformer T1. The induced current in the secondary winding 12 is sensed by the control function 16A and opens the DPST switch 16B, thereby interrupting the leakage current as well as the power to the load 14. In a typical GFI circuit 10, once the circuit interrupter 16 trips (the DPST switch 16B opens), the leakage path must be cleared, and a user must manually reset the switch 18B to the closed state.
In order to permit a user to verify that the GFI circuit 10 is operational, i.e., that an apparent zero primary winding current and/or zero secondary winding current is not due to a malfunction, the GFI circuit 10 includes a test feature. The test feature is implemented via a test button or witch S1, which may be manually pressed by the user. Pressing the test switch S1 is intended to cause the circuit interrupter 16 to trip and open the DPST switch 16B, thereby indicating that the GFI circuit 10 operates properly. Pressing the test switch S1 causes a small resistive load R1 to draw a current that bypasses the primary winding of the transformer T1. The bypass current has the same effect as leakage current in that the sum of the respective AC currents through the phase and neutral wires L, N, passing through the primary winding of the transformer T1 is non-zero. The non-zero current through the primary winding induces a current in the secondary winding 12 of the transformer T1, which is sensed by the control function 16A and opens the DPST switch 16B, thereby interrupting current paths over the phase and neutral wires L, N.
While leakage current is of concern in Information Technology (IT) equipment rooms, the conventional GFI circuit 10 is not a suitable solution to leakage current problems. IT equipment rooms (also known as data centers) utilize hundreds or even thousands of units of IT equipment. Each piece of IT equipment receives primary AC power by plugging into an outlet of a power distribution unit (PDU). A PDU is also a piece of IT equipment and typically includes: (a) a high power inlet from which power is received (typically from a panel board); (b) multiple lower power outlets; and (c) (optional) circuit breakers or fuses to protect the outlets from over current conditions (short circuits, etc.). PDUs are often designed to report certain status information over a communication and/or input/output interface, including: (a) the voltage being supplied to a given PDU's inlet, (b) how much power is flowing in the inlet and each outlet, and (c) the trip state (whether voltage is present) of each circuit breaker.
In a data center, it is not practical to use a standard GFI circuit 10 for a number of reasons. For example, it would be far too disruptive to unconditionally interrupt AC power to an IT device due to leakage current. Indeed, sensitive data may be corrupted and/or irrevocably lost if AC power were interrupted without notice. Additionally, industrial equipment (such as in data centers) may not be permitted to exhibit the same level of leakage current as those established in conventional residential GFI circuits 10. Indeed, the permissible level of leakage current in a data center (and/or other industrial environment) may be up to about 3.5 mA of current. Standard GFI circuits, however, may employ a fixed trip threshold for leakage current that is not appropriate for a data center. Still further, since data centers include thousands of units of IT equipment, it would be far too time-consuming, and susceptible to error, to manually test each GFI circuit 10.
Although the prior art systems address some issues associated with leakage current, the known solutions are unsatisfactory in the context of a data center (or other industrial environment). There are, therefore, needs in the art for new methods and apparatus for sensing ground leakage current in a system, and more particularly, for sensing such ground leakage and also operating to perform self-testing to ensure proper sensing capability.